Laser processing offers several advantages in terms of efficiency enhancement and manufacturing cost reduction for high-performance, high-efficiency solar cell processing. Firstly, advanced crystalline silicon solar cells may benefit from having the dimensions of the critical features such as electrical contacts be much smaller than the current industrial practice. For front contacted solar cells the contact area of the front metallization to the emitter as well as the contact area of the back metal to the base needs to be low (or the contact area ratios should be fairly small, preferably much below 10%). For an all back-contact, back-junction solar cell, where the emitter and base regions forming the p/n junction and the metallization are on the same side (the cell backside opposite the sunny side), the dimensions of the various features are typically small for high efficiency. In these cells where typically the emitter and base regions form alternate stripes, the width of these regions (in particular the width of the base contact) tends to be small. Also, the dimensions of the metal contacts to these regions tend to be proportionally small. The metallization connecting to the emitter and base regions then needs to be patterned to a correspondingly finer scale. Generally, lithography and laser processing are the technologies that have the relatively fine resolution capability to provide the small dimensions and the control required. Of these techniques, only laser processing offers the low cost advantage required in solar cell making. While lithography requires consumables such as photoresist and subsequent resist developer and stripper (which add to the process cost and complexity), laser processing is a non-contact, dry, direct write method and does not require any material consumables, making it a simpler and lower cost process for solar cell fabrication. Moreover, laser processing is an excellent choice for environmentally benign manufacturing since it is an all-dry process which does not use any consumables such as chemicals.
Further, to reduce the cost of solar cells there is a push to reduce the thickness of the crystalline silicon used and also at the same time increase the cell area for more power per cell and lower manufacturing cost per watt. Laser processing is suitable for these thin wafers and thin-film cell substrates as it is a completely non-contact, dry process and can be easily scaled to larger cell sizes.
Laser processing is also attractive as it is generally a “green” and environmentally benign process, not requiring or using poisonous chemicals or gases. With suitable selection of the laser and the processing system, laser processing presents the possibility of very high productivity with a very low cost of ownership.
Despite these advantages, the use of laser processing in crystalline silicon solar cell making has been limited because laser processes that provide high performance cells have not been developed. Disclosed here are laser processes using schemes that are tailored for each key application to produce solar cells with high efficiency. Specific embodiments are also disclosed for applications of laser processing in manufacturing thin-film crystalline silicon solar cells, such as those manufactured using sub-50-micron silicon substrates formed by epitaxial silicon growth.